• Heavy Rain/Flood NV/AZ/UT

    From Mike Powell@618:250/1 to All on Sat Aug 17 09:36:00 2024
    AWUS01 KWNH 171404
    FFGMPD
    AZZ000-UTZ000-NVZ000-171900-

    Mesoscale Precipitation Discussion 0891
    NWS Weather Prediction Center College Park MD
    1004 AM EDT Sat Aug 17 2024

    Areas affected...southeast Nevada, northwest Arizona, southern
    Utah

    Concerning...Heavy rainfall...Flash flooding possible

    Valid 171403Z - 171900Z

    Summary...Showers and thunderstorms will expand across the eastern
    Great Basin and into the Four Corners through the morning.
    Rainfall rates are expected to increase to 0.5-1"/hr, leading to
    hourly rainfall accumulations of up to 0.75". Flash flooding is
    possible.

    Discussion...The GOES-E WV imagery this morning shows a potent
    shortwave lifting across far southern NV, accompanied by expanding
    reflectivity associated with deepening showers and thunderstorms.
    The environment into which this shortwave is lifting is becoming
    increasingly favorable for heavy rainfall as reflected by 3-hr
    MUCAPE change from the SPC RAP reaching +200 J/kg combined with a
    slow surge of PWs of 1-1.3" lifting into the northern High Deserts
    of NV and far SW UT. Further evidence of the amplifying
    environment is rising lightning-cast probabilities coincident with
    increasing glaciation noted in the GOES-E day cloud phase RGB, and
    it is likely that the next few hours will feature a rapid
    expansion and intensification of reflectivity across the region.

    As the shortwave continues to pivot northward, it will combine
    with increasing bulk shear and enhanced diffluence within the RRQ
    of a strengthening jet streak arcing into the Intermountain West.
    Together this will drive strong deep layer ascent, which will work
    across robust thermodynamics as daytime destabilization occurs in
    conjunction with a persistent surge of low level moisture from the
    south, and 850-700mb moisture flux anomalies are progged to reach
    nearly +5 sigma according to the SREF. This will not only cause
    expanding convective coverage, but intensification of moist
    updrafts leading to rainfall rates which have a 10-20% chance of
    exceeding 1"/hr according to the HREF. Mean 850-300mb winds will
    remain progressive through the afternoon at 20-30 kts or more,
    reducing the residence time of any individual cell, but hourly
    rainfall of 0.5-0.75 inches is still likely in some areas as noted
    by the HRRR 15-min rainfall product and UA WRF hourly rain
    accumulations. Additionally, Corfidi vectors aligned to the mean
    0-6km wind suggest repeating or training rounds of storms is
    likely, which could produce 1-2" of rain with locally more than 3"
    possible (HREF neighborhood probabilities for 3"/6hrs 10-15%).

    Despite the fast motion, these intense rainfall rates, combined
    with any training, could quickly overwhelm soils across the area.
    Recent rain has increased the 0-10cm soil moisture percentiles to
    as high as 90% in some areas according to NASA SPoRT, increasing
    the vulnerability of what is already a sensitive region to heavy
    rain rates due to the many slot canyons, dry washes, and recent
    burn scars across the area. Any heavy rain rates moving across
    these areas could quickly result in rapid runoff and flash
    flooding.

    Weiss


    ATTN...WFO...FGZ...LKN...SLC...VEF...

    ATTN...RFC...CBRFC...CNRFC...NWC...

    LAT...LON 40291288 40121105 39141042 37781083 37081144
    36471263 36091461 36111579 36611625 37231615
    37991574 39151493 39931420
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